I discovered once that when the skin of the scrotum is slightly stretched (until it is shiny) it becomes much more sensitive to pleasurable stimulation. Whether by the other hand, if you are touching yourself, or by another's hand, lips, tongue or nipple, use a very light brushing stroke for the most pleasure. This helps to bring on or restore an erection and is an excellent way to float in ecstacy and to show gentle affection to your male partner.
When I am caressing myself I reach down with my left hand, palm toward the body and grasp under the sac gently, placing the thumb between the penis and the testes. Gradually, gently close the thumb toward the hand and pull slightly outward until you are holding your sac like a boquette of flowers. Very gently squeeze until the skin of the sac has a satin sheen. At this point a gentle brushing with your fingertips will produce an 'Aaahhh.'
This technique is great for beginning lovers who are exploring each other's bodies and thrilling to the new intimacy. You can easily go on to self or mutual masturbation from there with a new sense of trust and bonding if you are with a partner.
Question to the ladies: Does gently stretching the outer labia have a similar heightening of sensitivity?